At present, attempts are being made to synthesize a large number of new imidazopyridazine compounds serving as effective drugs against a variety of diseases. For example, EP-0,381,130 describes a compound represented by the formula: ##STR3## wherein R.sup.1 represents a hydrogen atom, a lower alkyl group which may be substituted, or a halogen atom; R.sup.2 and R.sup.3 independently represent a hydrogen atom, a lower alkyl group which may be substituted, a cycloalkyl group which may be substituted, or a phenyl group which may be substituted; R.sup.2 and R.sup.3 may cooperate with the adjacent nitrogen atom to form a heterocyclic ring which may be substituted; X represents an oxygen atom or S(O).sub.n (n is an integer from 0 to 2); Alk represents a linear or branched C.sub.1-10 alkylene, provided that X is an oxygen atom, when (i) R.sup.1 is a hydrogen atom, and (ii) either R.sup.2 or R.sup.3 is a hydrogen atom, the other being a hydrogen atom or a lower alkyl, and (iii) Alk is a linear C.sub.2-4 alkylene, or a salt thereof. EP-0,440,119 describes a compound represented by the formula: ##STR4## or a salt thereof.
EP-0,444,549 describes a compound represented by the formula: ##STR5## wherein R.sup.1 represents a halogen atom or a lower alkyl group which may be substituted; R.sup.2 and R.sup.3 independently represent a hydrogen atom, a lower alkyl group which may be substituted, a cycloalkyl group which may be substituted, or a phenyl group which may be substituted; R.sup.2 and R.sup.3 may cooperate with the adjacent nitrogen atom to form a heterocyclic ring which may be substituted; X represents an oxygen atom or S(O).sub.k (k is an integer from 0 to 2); the --A-- group is a divalent 3- to 7-membered homocyclic or heterocyclic ring; m.sup.1 and m.sup.2 independently represent an integer from 0 to 4 and n represents an integer from 0 or 1, or a salt thereof. These compounds are shown to possess antiasthmatic activity.